1. Field of the Invention
The present invention is directed to a local antenna for transmitting and/or receiving radio frequency signals in a nuclear magnetic resonance imaging apparatus, of the type having first and second axially spaced sub-antennas, with each sub-antenna being formed by at least one conductor turn.
2. Description of the Prior Art
A local antenna of the aforementioned type, having axially spaced first and second sub-antennas, with each sub-antenna being formed by at least one conductor turn, is described in German OS 36 28 035. The local antenna disclosed therein is specifically for nuclear magnetic resonance imaging, with the two sub-antennas being secured to a stand. Each sub-antenna, constituting a coil, has a variable capacitor connected thereto, with the variable capacitors for the respective sub-antennas being set to respectively different capacitances, thereby forming two different sets of resonant conditions. The two sub-antennas are connected in parallel, and form a Helmholtz arrangement wherein the distance between the coils is prescribed by their diameters. As a result, this known local antenna cannot be adapted as well as desired to the anatomy of a body part to be examined. The signal-to-noise ratio is therefore degraded, in some instances due to a filling factor for the local antenna which is too low.
Another local antenna is disclosed in U.S. Pat. No. 4,733,190. This local antenna design includes a base carrier on which two local coils are arranged axially spaced from each other. Each local coil is composed of two so-called "loop-gap" resonators connected oppositely in parallel, with the local coils being connected in parallel via the base carrier. The spacing of the two local antennas from each other can be set, dependent on the size of the body part to be examined, so that the anatomical structures of a hand as well as a thigh or a head can be covered with the local antenna design. This is accomplished by plugging the respective local antennas into the base carrier in sets of receptacles in the base carrier which are differently spaced from each other. Because the resonant frequency of the loop-gap resonator is principally defined by its geometry, the manufacture and formatting are relatively complicated, because tight tolerances must be observed. Moreover, loop-gap resonators can generate undesired oscillatory modes, which are difficult to suppress. If such modes are incompletely suppressed, unwanted coupling of the antennas to the excitation system can arise.